The sea water cell described consists of an anode made from an electronegative alloy e.g. based on magnesium, zinc, aluminum or lithium, and the cathode is a more or less inert current conductor. Common materials in the cathodes are materials which are resistant to sea water, such as copper, stainless steel, titanium or carbon. The cathode may also be coated with a catalyst which catalyzes the reduction of oxygen. Sea water contains little oxygen, about 10 g/m.sup.3. As a result of this the oxygen reducing sea water cells must have a very open structure in order to allow sufficient flow of fresh sea water through the cathode. Batteries consist usually of cells which are connected in parallel because the cells have the seawater as a common electrolyte. The common electrolyte would give short circuit currents via the sea water in series connected batteries. A DC/DC converter converts the low voltage (1 to 2 V) of the sea water cell to a more useful value, as e.g. 28 V.
In a battery having magnesium anodes the following reaction will take place:
At the anode: 2 Mg=2 Mg.sup.2+ +4 e.sup.-
The electrons liberated at the negative electrode are consumed at the positive electrode (the cathode): EQU O.sub.2 +2 H.sub.2 O+4e.sup.- =4 OH.sup.-
The concentration of oxygen in sea water is low, so that the transport of oxygen to the surface of the cathode will be the reaction step limiting the performance of the battery. Further one has to ensure that the surface of the electrode does not become so alkaline that it leads to deposition of calcium carbonate from the sea water, as this may form a layer on the cathode. Such a layer will, if it is formed, lead to a permanent reduction of the performance of the battery. For this reason the alkalization must be limited. This is obtained by limiting the current so that it does not exceed a certain percentage of the limiting current of the cathode. The limiting current is the current density at which the concentration of oxygen at the surface of the electrode is zero, in other words where the current density is so high that any oxygen molecule which is transported to the electrode surface by diffusion or convection, is reduced by formation of hydroxyl ions. The cathode is therefore given such a structure that a highest possible limiting current is obtained. A different parameter which is often used in literature is the so called mass transfer coefficient, k.sub.m, which in this case is the limiting current density divided by the oxygen concentration.
The limiting current density increases with increased water velocity, oxygen concentration and temperature and decreases with increased size of the cathode element. It has been found to be advantageous to make the cathode from expanded metal, net or metal wool because this can limit the size of the cathode in the flow direction of the sea water. The cathode can also be designed so that there is little resistance against flow-through, so that the sea water within the cathode is renewed continuously. These problems and solutions are described in int. pat. appl. PCT/N090/00045.